Hitherto, as photoconductive materials of photoreceptors used in electrophotography, inorganic substances such as, for example, selenium, cadmium sulfide, zinc oxide and silicon have been known, and extensively studied and practically utilized. These inorganic substances have many advantages, but also have various defects. For example, selenium has the defects that the production thereof is difficult and it readily crystallizes with heat or mechanical shock, and cadmium sulfide and zinc oxide have problems in moisture resistance and endurance. Silicon is said to have difficulties in chargeability and in production thereof. In addition, selenium and cadmium sulfide have a problem of toxicity.
On the other hand, organic photoconductive materials have the advantages of better film-forming ability and flexibility, lighter weight, higher transparency, and easier design of photoreceptors for wide wavelength region using proper sensitizing methods. Thus, they are increasingly attempted to put to practical use.
Fundamental properties required for photoreceptors used in electrophotography are (1) high chargeability when exposed to corona discharge in the dark, (2) less leakage, in the dark, of the charge obtained (decay in the dark), (3) rapid release of the charge when exposed to light (decay with light), and (4) little residual charge after exposure to light.
Many researches have been conducted on photoconductive polymers such as polyvinylcarbazole to be used as organic photoconductive substances. These compounds, however, are not satisfactory in film-forming properties, flexibility and adhesiveness and, besides, are difficult to fully satisfy the above mentioned fundamental requirements.
As for organic low molecular photoconductive compounds, photoreceptors excellent in mechanical strength such as flexibility and in film-forming properties and adhesiveness can be obtained by selecting binders used for the formation of photoreceptors, but it has been difficult to find compounds suitable to produce photoreceptors capable of maintaining high sensitivity.
In order to improve these problems, organic photoreceptors of higher sensitivity have been developed in which charge generation function and charge transport function are performed by separate substances. The feature of this so-called function-separated type photoreceptors is that substances suitable for the respective functions can be selected from a wide variety of substances and photoreceptors having optional performances can be easily produced. For this reason, many researches have been made thereon.
Various substances such as phthalocyanine, squarylium dyes, azo pigments, perylene pigments and others have been studied as substances which perform charge generating function. Among them, azo pigments can have various molecular structures and can be expected to give a high charge generating efficiency, and have been widely studied and gradually put to practical use. However, the relation between molecular structure and charge generating efficiency has not yet been clarified, and at present, a great number of studies on synthesis have been conducted to search for the optimum structures.
On the other hand, substances which perform charge transporting function include hole transport substances and electron transport substances. Various substances such as hydrazone compounds and stilbene compounds have been studied as the former substances, and 2,4,7-trinitro-9-fluorenone, diphenoquinone derivatives, and others have been studied as the latter substances, and these are attempted to put to practical use. Also as to these substances, a great number of studies on synthesis have been conducted to search for the optimum structures. In fact, various improvements have been made, but those which satisfy the fundamental properties required as photoreceptors mentioned above and have high endurance have not yet been obtained.
As mentioned above, various improvements have been made in production of electrophotographic photoreceptors, but, at present, those which satisfy the fundamental properties required as photoreceptors mentioned above and have high endurance have not yet been obtained.